I made the mistake of installing an AFR meter on my 912UL, to try to understand the vibration issue I had at cruise. Engine does not have an airbox, but KN-filters because of space issues. The AFR meter shows a very lean condition at cruise afr 15.5
I tried rising the needles to richer and it made it better below maybe 5000 but still very lean running at cruise. I then tested with larger needle jets and found that with 2.80 jet needle it is fine at cruise, but too rich further down in the register. I am happy the main jet is correct as I get AFR 12.5 at WOT climb out.
With that I concluded the issue is the CV carbs don't work properly without an airbox. I have btw had the same with motorbikes. Remove airbox and it never runs good with CV carbs. Google is full of people discussing this issue.
So I tried cutting down the slide springs and that helped. I have done it in increments and today I was back at 2.74 needle jets (just one above stock) and needle in richest position. It still runs lean at cruise at 15.2+ AFR which is not acceptable, but it only takes a tine bit of throttle and it starts richening up. I
It used to be lean all the way then apply full throttle and suddenly with a marked change AFR drops to a correct 12.5. With the gradual cutting back on the spring the transition has gotten progressively smoothed out and now, with a full 4 spring windings removed the mixture drops to below 15 with only slightly more throttle. Tmrw I am thinking take off another 1/2 coil. Anyone who has tried this. No comments please on put it back to stock. It is worth exploring
